1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to systems and methods for cooling solid-state laser media.
2. Description of the Related Art
As is well-known in the art, lasers emit waste energy in two forms: 1) sensible heat and 2) fluorescence. Sensible, heat is energy that is conducted through the surface while fluorescence is waste light that is created within the medium. Sensible heat is produced by non-radiative processes in the laser medium that enable relaxation of the excited species, which may be rare-earth ions in a crystal, from a higher energy level to a lower energy level. An unavoidable portion of the sensible heat is due to the xe2x80x9cquantum defectxe2x80x9d which is the difference in photon energy between the pump light and the laser light. Other processes may contribute to the sensible heat, including photon-assisted quenching of the upper state population by intermediate energy levels, upconversion and subsequent non-radiative relaxation, and ground-state absorption. Fluorescent light is an omni-directional spontaneous emission resulting from incomplete extraction by the laser beam of the energy stored in a population inversion. Fluorescent light does not create heat until it impinges upon a surface and is absorbed.
Waste energy is particularly problematic with respect to solid-state lasers. That is, chemical lasers, gas lasers, and liquid medium lasers, such as dyes, have a lasing medium which may be caused to flow out of the active region of the laser cavity for cooling. Solid-state lasers, on the other hand, typically have a stationary lasing medium, which does not allow heat removal by forced convection of the lasing medium. That is, once the medium is heated with pump energy, it cannot simply be removed from the pump cavity for cooling. Unfortunately, failure to successfully remove heat from solid-state lasers will cause the temperature of the medium to substantially increase, and may degrade the operation of the laser and reduce its reliability.
For slab laser media that are cooled across the broad face of the slab, the sensible heat may be transported through the thin dimension to the slab""s surfaces by conduction without incurring excessive temperature drops. The radiant energy of spontaneous emission, however, must also be rejected. In some solid-state lasers, such as ytterbium-doped yttrium aluminum garnet (Yb:YAG), the energy of the fluorescent radiation component may exceed that of the sensible heat component by a factor of four.
Accordingly, U.S. patent application Ser. No. 10/202,752, filed Jul. 25, 2002, by A. Betin and W. Griffin and entitled MULTI-JET IMPINGEMENT COOLED LASER PUMPHEAD AND METHOD (Attorney Docket No. PD-01W090), the teachings of which are incorporated herein by reference, discloses and claims an invention which uses liquid-phase multi-jet impingement directly on the broad face of a slab to remove the sensible heat component and either uses an absorbing species in the cooling liquid or an absorbing structure immersed within the cooling manifold to absorb the fluorescent component in a distributed volume at some distance away from the slab face, such that the fluorescent heating does not add to the sensible heat at the slab face. This approach provides excellent thermal transfer at the surface, avoids problems associated with nucleate boiling in a 2-phase cooling system and is compatible with a continuously operating high power laser system.
However this approach also relies on the slab pumphead cooling loop to remove both the sensible and fluorescence components. This places an additional thermal load on the coolant and the associated heat transfer equipment which rejects heat from the coolant, requiring high flow rates and large diameter cooling lines in an already congested volume. This may increase the requirements on the external heat dissipator by a factor of five. For a spaceborne application which rejects heat by radiation to ambient, this increase may become critical.
Thus, there is a need in the art for an improved, inexpensive, lightweight system or method for removing waste energy from lasers, particularly solid-state lasers, without converting the fluorescent component of the waste energy to heat.
The need in the art is addressed by the laser cooling apparatus and method of the present invention. Generally, the inventive apparatus includes a mechanism for transporting thermal energy from a laser and for communicating fluorescent radiation therefrom as well.
In the illustrative embodiment, the apparatus includes an optically transparent cooling structure or manifold with an inlet port, an exhaust port and a plurality of spray nozzles therebetween adapted to direct a cooling fluid on the active medium of a solid-state laser. In the best mode, the manifold is constructed of sapphire, notwithstanding the fact that diamond and other optically transparent materials may be used, such that the fluorescent component of waste energy is transmitted as light out of and away from the laser pump cavity (or pumphead).